Two-phase cobalt iron alloys prepared by powder metallurgy

ABSTRACT

A DUCTILE COBALT ALLOY CONTAINING ABOUT 1.5 TO ABOUT 15% BY WEIGHT IRON AND BALANCE COBALT AND BEING FABRICATED BY POWDER METALLURGY METHODS FROM COBALT AND IRON POWDERS. THE ALLOY HAS A TWO-PASE MICROSTRUCTURE IN WHICH THE IRON CONTENT OF ONE PHASE IS HIGHER THAN THE IRON CONTENT OF THE OTHER PHASE AND HAS A PREDOMINANTLY FACE-CENTERED CUBIC CRYSTAL ALLOTROPIC STRUCTURE AND SUPPRESSED TRANSFORMATION FROM SAID FACE-CENTERED CUBIC CRYSTAL STRUCTURE TO A HEXAGONAL CRYSTAL STRUCTURE AT ROOM TEMPERATURE.

Dec. 14, 1971 B. w KUSHNIR ETAL 3,626,570

TWO-PHASE COBALT IRON ALLOYS PREPARED BY POWDER METALLURGY Filed Sept. 19. 1969 2 Sheets-Sheet 1 C010 Fe Inventors BUD W KUSHNIR ROBERT W. FRASER DAVID J. I. EVANS lc IRON CONCE NTRATION Dec. 14, 1971 B. w KUSHNIR ETAL TWO-PHASE COBALT IRON ALLOYS PREPARED BY POWDER METALLURGY Filed Sept. 19. 1969 DISTANCE, MICRONS FIG. 2

- CUP DUCTILITY .32 :48 30 44 2 40 .265 36 24 :32 .22 :28 .20 :24

18- BEND .16-

2 Sheets-Sheet 8 FIGB I N VEN'IOR. BUD W. KUSHNIR ROBERT W. FRASER DAVID J. I. EVANS United States Patent 3,626,570 TWO-PHASE. COBALT IRON ALLOYS PREPARED BY POWDER METALLURGY Bud W. Kushnir, Edmonton, Alberta, Robert William Fraser, Fort Saskatchewan, Alberta, and David John Ivor Evans, North Edmonton, Alberta, Canada, assignors to Sherritt Gordon Mines Limited, Toronto, Ontario. Canada Filed Sept. 19, 1969, Ser. No. 859,252

Claims priority, application Canada, Nov. 15, 1968,

35,250 Int. Cl. B221? N00 US. Cl. 29-182 2 Claims ABSTRACT OF THE DISCLOSURE A ductile cobalt alloy containing about 1.5 to about 15% by weight iron and balance cobalt and being fabricated by powder metallurgy methods from cobalt and iron powders. The alloy has a two-phase microstructure in which the iron content of one phase is higher than the iron content of the other phase and has a predominantly face-centered cubic crystal allotropic structure and suppressed transformation from said face-centered cubic crystal structure to a hexagonal crystal structure at room temperature.

This invention relates to ductile cobalt-iron alloys workable at room temperature and is particularly directed to cobalt-iron alloys produced from metal powders by powder metallurgy methods.

It is known, as stated in Canadian Pat. No. 669,381, issued Aug. 27, 1963, that cobalt normally has a closepacked hexagonal (HCP) crystal structure at room temperature and coldforming of cobalt having such a crystal structure produces a workhardening or build-up of internal stresses which decreases ductality and prohibits further cold-forming until a stress-relieving heat treatment is applied to the metal. It is also known that at temperatures above about 780 F. cobalt has a face-centered cubic (FCC) crystal structure which would permit cold-forming if this crystal structure existed at room temperatures, but an allotropic transformation in crystal structure from the cubic form to the hexagonal form takes place as cobalt cools below 780 F., thereby precluding working of the cobalt into thin sheet or tubing by cold rolling, swaging and the like.

It is also known, for example from Canadian Pat. No. 669,381, to alloy cobalt with an element selected from the group consisting of iron, carbon and nickel in amounts effective to retard the transformation from the HCP to the FCC crystal structure at temperatures below 780 F. and thereby provide a cold-formable, high cobalt alloy. According to these known procedures, the cobalt is alloyed with the additive metal by melting by standard furnacing procedures. Vacuum melting is required where it is necessary, as it usually is, to avoid contamination of the alloy by oxides and gas inclusions. As a result of the high melting point of cobalt and the high capital and operating cost of vacuum melting equipment, the cost of known high purity ductile cobalt-iron alloys is relatively high.

It is a principal object of the present invention, there fore, to provide low cost ductile cobalt-iron alloys.

It is another object of the present invention to provide cold-formable cobalt-iron alloy compositions having a novel microstnicture and capable of being readily coldworked into strip, sheet, wire and tubing.

These and other objects of the invention are achieved by utilizing powder metallurgy methods to fabricate a novel form of cobalt-iron alloy from mixtures of cobalt and iron powder.

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We have found, surprisingly, that cobalt containing from about 1.5% to about 15% iron, preferably about 5-7% iron, fabricated by powder metallurgy methods, will retain a FCC crystal structure at room temperature even though the iron is not uniformly alloyed with the cobalt as is the case with conventional melted and cast cobalt-iron alloys. That is, we have found that, quite unexpectedly and unlike other alloys prepared by powder metallurgy methods, cobalt-iron alloys so prepared do not require an expensive homogenization heat treatment to develop the alloy properties found in comparable homogeneous alloys produced by melting and casting methods. Accordingly, because of the ready availability of high purity iron and cobalt powders, the overall economies realizable with powder metallurgy methods and the relative ease with which contamination is avoided in powder metallurgy methods, the novel non-homogeneous cobaltiron alloy compositions of the invention may be etficiently and economically produced, for example, by direct powder rolling followed by sintering, working and annealing.

The invention and the manner it is carried into practice is explained in the following detailed description with reference to the accompanying drawings, in which.

FIG. 1 shows the miscrostructure of cobalt-iron alloys of the present invention X250 magnification) containing 3%, 7% and 10% by weight iron;

FIG. 2 is a graph showing an electron microprobe analysis across a strip of a Co7% Fe alloy according to the invention;

FIG. 3 is a graph illustrating the effect of iron content on the tensile properties of cobalt-iron alloys according to the present invention; and

FIG. 4 is a graph illustrating the effect of iron content on the drawing and forming properties of cobaltiron alloys according to the present invention.

In general, the alloy compositions of this inventionare prepared by applying conventional powder metallurgy fabrication methods to blends of cobalt and iron powders. According to these conventional procedures, metal powders are first compacted either by static pressing to form billets or by dynamic pressure such as direct powder rolling. The green compact is then sintered, usually in a controlled, non-oxidizing atmosphere and the sintered compact is then hot worked, such as by extrusion, forging or rolling, to densify the workpiece to substantially of theoretical density. The dense workpiece may then be treated by any combination of hot and/or cold working and annealing steps to produce a final wrought product of the desired shape and properties.

Certain specific procedures and conditions must be observed in applying these general powder metallurgy methods is the preparation of the cobalt-iron compositions of the present invention. The starting powders should be of high purity and preferably should have a particle size below about 300 microns and a normal size distribution. Commercially available cobalt powder produced by hydrogen reduction from aqueous ammoniacal cobalt salt solutions is ideally suited for the invention as are commercially available iron powders produced, for example, by the carbonyl process.

The cobalt-iron powder blend is prepared by thoroughly mixing from about 1.5 to about 15% by weight iron powder, preferably about 5-7% iron powder, with the cobalt powder. A small amount, e.g. up to 2% of water or other volatile, non-residue forming liquid, may be mixed with the powder blend to improve its compacting characteristics.

Since the alloys of this invention are used in the form of strip or sheet, a preferred compacting procedure is direct powder rolling. Suitable direct metal powder rolling methods and apparatus are described in detail, for exarn ple, in Canadian Pat. No. 675,888. Compacted cobalt-iron strip having a density in the range of 60 to 90% and having high green strength can be readily produced in the nanner described in that patent from blends of commerzially available cobalt and iron powders.

The green compact or strip is sintered on a continuous or batch basis at a temperature in the range of about 1400 F. to about 1900 F., preferably under a flowing Wet hydrogen atmosphere to elfect partial consolidation of the metal particles and to lower sulphur and carbon contaminants in the powder blend to below about 0.005% C and 0.010% S respectively. Normally, about one to about four hours sintering will be required.

The sintered strip is next hot-rolled at an initial strip temperature of 1700 F. to 1900 F. preferably to take a 20-40% reduction in cross-sectional area and to increase the density to about 100% of theoretical. During rolling, preferably the incoming hot strip is protected from oxidation by a surrounding atmosphere of hydrogen. Exittng strip may be allowed to cool in air.

The hot rolled strip is annealed for a short period, preferably about 30 minutes, at a temperature in the range of 1200 F. to 1600 vF., preferably about 1400 F., and is then cold rolled again preferably with a 20% to 40% cross-sectional area reduction. The cold rolled strip is then given a final anneal at about 1600 F. to about 1900 F.

The product cobalt-iron strip has a high room temperature ductility and is characterized by a novel non-homogeneous, two phase 'microstructure in which the iron content of one phase is higher than the iron content of the other phase.

To further illustrate the invention, preferred embodiments of the invention will now be described by reference to specific experimental results which have been obtained.

A series of cobalt-iron alloys containing up to 15% by weight iron was prepared using powder metallurgy laboratory techniques. The alloys were made by mixing commercially pure cobalt and iron powders, roll compacting the blended powders into green strip, directly hot rolling the roll-compacted strip into fully dense alloy strip, followed by annealing and cold rolling steps.

High purity cobalt powder produced by hydrometallurgical techniques employing hydrogen reduction from an aqueous system and having an irregular shape and rough TABLE 1.COBALT AND IRON POWDER PROPERTIES Cobalt Iron powder powder General Shcrritt Aniline Gordon HP S grade grade Chemical analysis, percent' N 0.074 N.d. 99. 9 N .(1. 0. 002 0. 002 0. 015 09. 9 0. 050 0. 040 0. 020 0.007 Screen analysis, percent:

mesh 50 mcsh 4.1 100 mesh. 14. 2 0.2 150 200 mesh 23. 0 0.3 -200 250 mesh 12. 1 1. 0 250 325 mesh. 16. 3 8.0 325 500 mesh. 17. 7 2.0 500 750 mesh. 10. 2 7. 4 750 2. 4 81. 1 Physical properties:

Apparent density, gm./cc 2. 49 2. 58 Flow rate (Hall), see/50 gm. 35. 7 Fisher Number 16. 5 4. 8

1 Trace. 2 Nil.

Five pound charges of the foregoing cobalt and iron powders were dry-blended for 30 minutes after which a 1% water addition was made to each charge and the wetted powder blended for an additional 10 minutes. The blended powder was roll-compacted into green strip by feeding the powder continuously into a roll gap opening of 0.01" formed by a pair of opposed rollers. The compacted green strip was maintained at 1600 F. in a sintering furnace for 2% hours in a continuously moving wet hydrogen atmosphere to lower the carbon and sulphur contents from 0.05% to 0.02% to about 0.004% and 0.006% respectively. After sintering, the strip temperature was raised to 1800 F. for 15 minutes prior to a hot reduction of about 30% using four inch diameter Working rolls. The density of the hot rolled alloy strip was about 98 100% of theoretical density. The strip was then annealed for 22 hour at 1400 F. in hydrogen, cold rolled with a 30% reduction in thickness and given a final an nealing treatment for /2 hour at 1800- F. The processed strip samples were about 0.02 inch thick.

Table 2 below tabulates the nominal chemical compositions of the Co-Fe alloys prepared and indicates their respective crystal structures as was quantitatively determined using X-ray diffraction techniques.

TABLE 2.EFFECT OF IRON CONTENT ON THE STRUCTURAL AND MECHANICAL PROP- ERTIES OF COBALT-IRON ALLOYS Cup duc- Nominal composition Structure Tensile properties tility, Bend cup spring Hard- Pereent Percent U.'I.S., Y.S. Elong, height, back," ness.* FC 0 HCP p.s.i. p.s.i. percent inches degrees DPI-I 123, 600 64, 800 16 156 50.8 226 128, 300 63, 100 23 161 50.5 N.d. 115,500 69,300 7 138 52. 8 N.d. 120,600 61,500 27 176 49. 5 N.d. 119, 400 62, 900 26 50. 3 N.d. 106, 900 N.d. 36 204 43. 5 N .d. 102, 300 N.d. 43 245 36. 7 177 93, 900 39, 500 53 314 28. 5 163 89, 800 34, 600 51 317 21. 8 163 85, 600 32, 200 56 339 17. 5 137 82, 600 100 53 323 17.0 133 78, 700 31, 300 54 333 16. 2 N.d. 76, 000 28, 500 50 333 15.0 128 74, 400 30,400 46 313 14.0 N.d. 71, 300 29, 500 46 315 14. 2 N.d. 72, 800 N.d. 44 317 N.d. 124

*The indicated values are the average of three determinations.

pebbly surface was mixed with carbonyl iron powder of General Aniline HP grade which had a nearly spherical shape and particle size smaller than that of the cobalt powder, thus permitting interstitial mixing with the relatively coarse cobalt powder. To illustrate the interstitial mixing, the apparent density of the blended cobalt-iron powder mixtures increased from 2.49 to 2.67 gm./cc. for a blend containing 10% of iron powder having an apparent density of 2.58 gm./cc. The characteristics of the cobalt and iron powder are shown in Table 1 below.

It will be evident from the table that the amount of transformed HCP allotrope decreased with increased iron content up to about 7% iron. At the upper range of iron contents, i.e. 7% up to 15% iron, no HCP phase was present in the alloy. At iron contents below about 2%, the HCP phase increased significantly minimizing the suppression of the FCC crystals to HCP crystals.

Each of the alloys was examined metallographically after processing and it was found that the alloys were chemically non-homogeneous and contained islands of a second phase. The amount of the second phase increased with increased iron content and typical microstructures are shown in FIG. 1 for 3%, 7% and iron contents in the cobalt. The Co-7% iron strip was examined and analyzed across its width with an electron microprobe which indicated that the second or island phase was ironrich and contained about 31% iron while matrix phase contained about 3% iron. The variation in iron content in the microstructure of this alloy along a random line across the strip is shown in FIG. 2. The composition of the iron rich phase suggests that it is (BCC) ferrite since it contains nearly an equilibrium amount of cobalt, i.e. about 70% Co. However, X-ray diffraction analysis of the Co- 7% Fe alloy indicated that the alloy consisted of FCC cobalt only and there was no evidence of the (BCC) iron structure.

The mechanical properties of each alloy in the experimental series were determined using conventional test specimens and procedures. The tensile properties were obtained using shaped tensile test specimens that had a twoinch gauge length and a one-half inch gauge width. The tensile tests were conducted at a constant cross head speed using a nominal strain rate of 0.25 in./in./min. The load-elongation curves were autographically recorded using an extensometer and the tensile elongation was determined by measuring the extension between scribed gauge markers after failure. The tensile properties (0.2% offset yield strength, ultimate tensile strength and tensile elongation) of each alloy are given in Table 2 and shown in FIG. 3 as a function of iron content.

The drawing and forming properties were determined using a modified Olsen cup test and the recommended ASTM procedure for measuring the permanent set and spring back after bending. The cup test consisted of drawing a cup in a two-inch square sheet specimen using a 0.587 inch diameter ball and then measuring the cup height when the sample fractured. The spring back test consisted of blending a 1" x 3" test specimen through a 90 bend using a two-inch test length (i.e. a relatively small bend radius) and then measuring the spring back after bending. The cup ductility (height of cup at fracture) and spring back after bending (degrees) of each experimental alloy are listed in Table 2 and shown in FIG. 4 as a function of iron content.

The hardness of the annealed alloys was also determined and the results are given in Table 2.

Table 2 and FIGS. 3 and 4 show that the mechanical properties of the experimental alloys depended upon iron content. As the iron content increased, the ultimate tensile strength, yield strength and hardness decreased and the tensile elongation, cup ductility and permanent set after bending increased. It was also determined that the rate of work hardening, however, was unaffected by iron content and the work hardening rate for all of the alloys in the series was essentially the same as that of pure cobalt.

The foregoing results show that cobalt-iron powder blends, containing up to iron. can be processed into ductile cobalt-iron strip using direct powder rolling procedures. Surprisingly, the iron additions to the cobalt increased ductility and suppressed FCCZHCP transformation and increased the amount of FCC allotrope despite the absence of cobalt-iron homogeneity which characterizes the microstructures of melted and cast cobalt-iron alloys.

In another experiments test, a cobalt alloy having a 7% iron content was tested on a commercial powder rolling line by the processing of 500 pounds of cobalt and iron powder having the composition indicated in Table 1. The powders were blended together and roll compacted into a 6 /2 inch wide x 0.07 inch thick green strip using a 10" diameter compacting mill. The compacted strip was coiled and fed into a continuous sintering and hot rolling line at 4 ft./min. where it was given a 25% thickness reduction at 1700 F. and then recoiled. After annealing at 1400 F. for /2 hour, the strip was given a 30% cold rolling reduction and then re-annealed at 1800 F. for /2 hour.

Table 3 below compares the structure and mechanical properties of the resulting material with those of the laboratory material described above. The microstructure was identical to the laboratory material and the mechanical properties were very similar to those of the laboratory material.

The cobalt-iron alloy of the present invention has particular utility in the manufacture of tubular, cobalt-base, hard-facing welding rods. In a specific example of this use, the hot rolled strip was annealed for /2 hour in hydrogen at 1400 F. cold rolled 30%, annealed 1800 F. for 15 minutes in hydrogen, cold rolled to 0.015" thick and the given a final anneal at 1800 F. for 15 minutes in hydrogen. The resulting strip was roll formed and filled with a blend of metal powders to yield a composite rod having the chemical composition of Stellite 6. The asformed tube rod was subsequently sWaged from 0.25" diameter to 0.13" diameter and no manufacturing difficulties (ductility or spring-back) were encountered during the forming or drawing operations.

What we claim as new and desire to protect by Letters Patent of the United States is:

1. A cold workable cobalt-iron alloy composition consisting essentially of from about 1.5 to about 15% by weight iron and balance cobalt and incidental impurities and having a predominantly face-centered cubic crystal allotropic structure and suppressed transformation from said face-centered cubic crystal to a hexagonal crystal structure at room temperature, said composition being fabricated from a mixture of cobalt and iron powders by powder metallurgy methods including powder compacting, sintering, hot and cold working and annealing, said composition having a non-homogeneous two-phase microstructure comprising a cobalt-iron alloy matrix phase containing islands of a second cobalt-iron phase which is richer in iron than the surrounding matrix phase.

2. A cobalt-iron alloy composition as claimed in claim 1 which consists essentially of from about 93% to about 95% cobalt and from about 5% to about 7% iron.

References Cited UNITED STATES PATENTS 5/1963 Faulkner 2919l.2 3/1970 Cape 29182 U.S. Cl. X.R. -170, 244 

